Dimethyl biphenyl (DMBP) compounds are useful intermediates in the production of a variety of commercially valuable products, including polyesters and plasticizers for PVC and other polymer compositions. DMBP compounds can be readily converted to an ester plasticizer by a process comprising oxidation of the DMBP to produce the corresponding mono- or dicarboxylic acid followed by esterification with a long chain alcohol.
For example, 4,4′-biphenyl-dicarboxylic acid, optionally together with 3,4′-biphenyl dicarboxylic acid, is a potential precursor, either alone or as a modifier for polyethylene terephthalate (PET), in the production of polyester fibers, engineering plastics, liquid crystal polymers for electronic and mechanical devices, and films with high heat resistance and strength.
Processes to produce DMBP compounds generally yield a mixture of all six DMBP isomers, namely 2,2′-, 2,3′-, 2,4′-, 3,3′-, 3,4′- and 4,4′-DMBP (see, for example, International Patent Application Publication No. WO 2015/112252).
However, for certain applications, it is desirable to maximize the purity of individual isomers, particularly the 3,3′-, 3,4′- and 4,4′-isomers.
Based on boiling point differences it is possible to separate the 3,3′-, 3,4′- and 4,4′-isomers from the 2,X′ isomers, where X=2, 3 or 4, utilizing, for example, fractional distillation. However, separation of the 3,3′-, 3,4′- and 4,4′-isomers from each other based on boiling point is more challenging, particularly separation of the 3,4′-isomer from the 4,4′-isomer which have very close boiling points (see Table 1 below).
TABLE 1IsomerNormal Boiling Point (° C.)Fusion Temperature (° C.)2,2′261192,3′2722,4′275−243,3′28983,4′293114,4′296115
Further, based on heat of fusion differences it is in principle possible to effect separation of 3,3′-, 3,4′- and 4,4′-isomers via crystallization. However, because the relative proportions of some of these isomers in a given mixture may be small, separation via crystallization may not be commercially attractive.
It is known that certain adsorbents, for example zeolites, can be used to separate individual hydrocarbons from mixtures thereof. Adsorptive separation may be useful where the components to be separated have similar physical properties such as boiling point and melting point. For example, utilizing zeolites it is possible to selectively separate a predetermined xylene from a mixture of xylene isomers. See, for example, United States Patent Application Publication No. 2009/0326310 and references therein.
In view of the above it would be desirable to provide alternative processes for the separation of 3,3′-, 3,4′- and 4,4′-DMBP isomers, particularly processes that may be amenable to commercial implementation.
The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgement or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.